This invention relates to the use of formaldehyde reactants in chemical processes or reactions, particularly those conducted in the presence of, or which generate some, water. In a preferred embodiment, this invention relates to a process for production of azomethines by reaction of an aniline with formaldehyde, and also to a process for production of haloacetanilides from anilines by reaction of the latter with formaldehyde to form azomethines, reaction of the azomethine with an acyl halide and, if an N,N-disubstituted haloacetanilide is the ultimate product, further reaction with an appropriate agent, for instance, an alcohol.
Processes of this general type, resulting in the formation of haloacetanilides are described, for example, in U.S. Pat. Nos. 3,630,716, 3,637,847 and 4,097,262. In all three patents, the first step involves the reaction of an optionally substituted aniline with formaldehyde to produce an azomethine according to the general reaction ##STR1##
In such processes, the physical state and nature of the formaldehyde can pose handling and recovery problems. Formaldehyde is generally commercially available in the form of either an aqueous solution, such as formalin, or solid paraformaldehyde. Formalin, being a liquid, is easier to handle than solids but the large amounts of water and alcohol associated with formalin pose disposal problems. Paraformaldehyde, being a polymeric solid, is less reactive and can form polymeric impurities. Furthermore, if removal of unreacted paraformaldehyde from the reaction mixture is required, filtration or sublimation at elevated temperatures would be necessary. These require expensive and high maintenance equipment on the one hand, or can produce degradation of the desired product (due to elevated temperature), as well as fouling of condenser tubes and other apparatus by deposited paraformaldehyde. Additionally, formaldehyde related impurities can be carried through the process, resulting in impurities in the ultimate product.
U.S. Pat. Nos. 3,630,716 and 3,637,847 describe (examples 36-47 of each) reaction of various anilines with formaldehyde used in the form of the trimer trioxymethylene. In some of these examples, the reaction is conducted in the presence of a small amount of trimethylamine in methanol, while in other examples, no trimethylamine or methanol was present. This appears to be explained by the comments in British Patent 1,078,072, that "basic catalysts" such as trimethylamine, may be used to neutralize any formic acid present in the formaldehyde. The patent points out, however, that the reaction will proceed in the absence of a catalyst and this is demonstrated by the presence of other examples in the two U.S. patents in which no catalyst was used.
Two more recent patents provide expedients to deal with the problems of formaldehyde handling referred to above.
U.S. Pat. No. 4,399,306 describes an overall process for production of N-alkoxyalkyl haloacetanilides involving a number of expedients, some of which are directed to the problems of formaldehyde handling. The patent points out that in the azomethine production step, formaldehyde is generally used in excess; that a stoichiometric amount of formaldehyde will not result in complete reaction of the aniline. The patent further refers to the problems of formaldehyde in the reaction products, including its appearance as an impurity in the azomethine as well as in the final haloacetanilide product. This patent proposes a process which uses a stoichiometric amount of an aqueous formaldehyde solution in an apolar solvent followed by dehydration to produce the azomethine derivative. The aqueous formaldehyde solution is removed at a temperature of 80.degree. C. to 140.degree. C. However, as pointed out above, the aqueous formaldehyde solution so removed must be disposed of.
U.S. Pat. No. 4,491,672 of Richarz et al. purports to solve the problems arising from use of formaldehyde by carrying out the production of the azomethine without a solvent, followed by distillation of the water at a pressure of less than 500 mbar, the distillation being carried out in the presence of an alcohol having a boiling point below 160.degree. C. In this process the formaldehyde is used in the form of a gas or a compound which "forms formaldehyde under the reaction conditions, e.g. paraformaldehyde or trioxane". Paraformaldehyde appears to be preferred.
The present invention, however, provides formaldehyde in a form which is more reactive and more usable, and which additionally can be recovered for reuse in the process.